“Three-dimensional structures” of these micrometric components mean micro-optoelectromechanical devices (MOEMS) or microelectromechanical (MEMS) devices, such as reed contactors, accelerometers, micro-motors, quartz resonators, sensors of micrometric size that have to be left to move freely after encapsulation in a controlled atmosphere. The construction of these three-dimensional structures of the micrometric components can occur on an insulating substrate or on a semi-conductor substrate in which integrated circuits have, for example, been made beforehand. In this latter case, it is possible to take the metallic contact pads of the integrated circuits to start the deposition of the metallic layers that will, in part, form the structure of the micrometric component and enable it to be electrically connected to said circuit.
EP Patent No. 0 874 379 by the same Applicant discloses a micro-contactor with micrometric sized strips as an example of a three-dimensional structure and the method of making the same. The contactor includes metallic strips that are at a distance from each other in the rest state and which are made by electrolytic means in several steps and secured to a substrate. The strips are formed of an iron and nickel alloy deposited by an electrolytic method. This alloy has the property of being ferromagnetic so that the strips can be put into contact with each other when a magnetic field passing through them creates an attraction force between them. At least one aperture or constriction is made on at least one of the strips to facilitate bending of the strip.
The method of checking hermeticity has to be able to detect a leakage rate from very small closed cavities of micrometric components whose volume is less than 1 mm3, for example of the order of 0.02 mm3. Long equalization time constants of at least 20 years must be guaranteed for the closed cavity of these micrometric components in order for the structure of each component to be protected from any contaminating fluid. The contaminating fluid can be a liquid or a gas.
When the cap of the micrometric component is fixed onto the substrate to enclose the structure to be protected, micro-cracks may be observed, liable to allow an external contaminating liquid or gas to penetrate the cavity and contaminate said structure.
In order to check hermeticity or detect leakage from a closed cavity of a component via a conventional method, a checking gas has to be introduced into the cavity. In order to do this, said component has to be placed in a container or an enclosure which is filled with a gas, such as helium at a high pressure in order to accelerate the introduction of the gas into said cavity. However, one drawback of this method is that the measuring gas introduced into the cavity is liable to escape partially or entirely if there is a large leak, which can distort the hermeticity check which is conventionally carried out using a mass spectrometer.
Another drawback of the conventional method is that the leakage detection threshold of a mass spectrometer system is of the order of 5·10−12 mbar·l/s. This means that the maximum leakage rates of the order of 10−15 to 10−14 mbar·l/s, which guarantee equalization constants of at least 20 years for cavities with a volume of less than 1 mm3, cannot be detected.
U.S. Pat. No. 6,223,586 discloses a method of inspecting leaks from electronic components, such as microelectromechanical devices having a closed cavity. In order to do this, plates of such electronic components are placed first of all in a liquid bath under pressure for a determined time period. The liquid under pressure can be water for example. Once this step is finished, the components are placed in another liquid and an inspection can be carried out via microscope to detect areas of leakage in the cavities of the components. This other liquid may also be water so as to prevent the water inside the cavity evaporating. Another major drawback of this method lies in the fact that the hermeticity checking sensitivity is greatly reduced.
The main object of the invention is therefore to overcome the drawbacks of the prior art by providing a cumulative type method of checking the hermeticity of a closed cavity of at least one micrometric component, with remarkably increased sensitivity. This method has the capacity to reveal large leakages in a single detection operation. Means are thus provided in each cavity in order to react with a quantity of fluid that has penetrated the cavity during a determined period of time for checking hermeticity.